Measured by the number of new applications being written for each type of device, Google’s (GOOG) Android operating system was closing the gap with Apple’s (AAPL) iPhone throughout the second half of 2009, according to Flurry Analytics.

Tracking new application starts, Flurry observed a 25% month-over-month growth in new Android projects over that period and, relative to Android, a slight slackening in the rate of new iPhone projects.

But in January, Apple surged ahead. In that month, Flurry saw the number of iPhone developers adding its tracking code to new projects jump three-fold over December.

“This represents the single largest spike in Flurry history, with over 1,600 new iPhone OS application starts for January,” wrote Flurry’s Peter Farago in a report posted Friday.

What caused the spike? Flurry attributes it to the imminent arrival of the iPad:

“Historically, Flurry has measured surges in new application starts within its system in anticipation of new device launches, including for the Motorola Droid and iPhone 3GS. As such, we hypothesize that excitement generated by Apple’s iPad event in January is driving this growth. For developers who get a jump on customizing their applications for the iPad, there may be an opportunity to stand out early on, and earn more downloads.”

We’ll find out in March whether the iPad excitement Flurry measured in January survived the unveiling of the device in February.

In January, Flurry used code embedded in its clients’ apps to detect the presence of 50 iPads being tested on the Cupertino campus in advance of their unveiling. See here.

[Follow Philip Elmer-DeWitt on Twitter @philiped]